This invention relates to the nuclear reactor art and has particular relationship to facilities for mounting or positioning, with integrity, the nuclear fuel elements or fuel rods of a reactor. It is essential that the fuel elements of each fuel assembly should be laterally supported so that they resist substantial lateral displacement. Displacement would result in nonuniform heat distribution along the fuel elements. There would be peaks of neutron flux and hot spots would develop.
Andrews U.S. Pat. No. Re. 28,079 discloses facilities for mounting and positioning fuel elements in accordance with the teachings of the prior art. As disclosed in Andrews, the lateral support is provided by grids composed of interfitted slotted straps which define cells or channels for the fuel elements. Each strap of one set is interlaced or crossed through its slots with the straps of a second set and each strap of the second set is interlaced or crossed through its slot with the straps of the one set. Each cell is bound by wall sections of the straps. One of a pair of opposite wall sections has a spring and the other has projections or dimples cooperative with the springs. The fuel element in a cell is resiliently urged laterally by the spring in one of each pair of wall sections into engagement with the projections in the opposite of each pair of wall sections and is thus effectively laterally supported. Usually there are a pair of vertically displaced dimples whose region of contact with a fuel element are symmetric with respect to the region of contact of the cooperative springs. To resist buckling and crushing under the pressures in a reactor, the grid structure is rendered rigid by metallurgically joining the crossed straps at their regions of intersection.
In the past practice as taught by Andrews, typically, the straps were composed of stainless steel. It was feasible to join the straps at their intersections by furnace brazing. Stainless steel has a moderately low neutron-absorption cross section (typically about 2 barns for thermal neutrons).
In the interest of improving the efficiency of nuclear reactors, it has become desirable to provide grid structure of straps of a material of substantially lower neutron-absorption cross section and for this purpose zirconium or ZIRCALLOY alloy (typically of a cross section of about 0.17 barns for the thermal neutrons) has been selected. Zirconium of ZIRCALLOY alloy cannot be metallurgically joined by brazing. It is accordingly necessary to form a rigid grid structure of straps of ZIRCALLOY alloy by welding the interfitted straps at their regions of intersection. Typically the joints may be formed by welding with a laser beam but there must be adequate metal to support the weld. The grid structure of Andrews does not lend itself to welding. There is not sufficient metal in the region of intersection to form a strong metallurgical joint. To thicken the metal overall would increment the weight and cost and parasitic neutron losses and also presents problems in the deflection out of the straps of the springs and dimples which are necessary to hold the fuel elements.
It is an object of this invention to overcome the disadvantages of the prior art and to provide a grid structure for laterally supporting fuel elements of a nuclear fuel assembly which shall lend itself readily to being formed into a rigid crush-resistant body by welding at the intersection regions of the straps and shall facilitate the deflection of holding dimples and springs for the fuel elements. It is also an object of this invention to provide a grid structure for laterally supporting fuel elements of a material which does not lend itself to brazing.